Facsimile overload control with nonlinear resistance in output



Dec. '28, 1948.

R. URTIS FACSIMILE OVERLOAD CONTROL WITH NONLINEAR RESISTANCE IN OUTPUT Filed Sept. 25, 1945 /3 6 I )7 J a '9 7 /3 sounc or. f M M 2 FAQs/Ma: 6/4/7445 1s a 5 M M mum 3+ 2 war-40mm HAS/STANCE FIG 1 C/MRACTEH/ST/CS a; Il/t' v 1 mam-40mm RES/STANCE FIG 2:

RC. CURTIS INVENTOR.

ATTORNEY Patented 1948' FACSIMILE ovantoan coN'raoL' wrrn. NONLINEAR assistance m ou'rro'r menu-a c. Curtia, sum mus,- N. 1,, aaaignor to Faximlle, Inc., New York,

of Delaware N. Y., a corporation Application September 25, 1949, Serial No. 899,171

The present invention concerns facsimile system and, in particular, electrolytic facsimile recording devices.

One object of the present invention is to pre-' vent overload in an electrolytic facsimile record-f 1 ing scanner.

Another object of the present invention is to prevent overload or burning of the recording sheet in an electrolytic facsimile recording scanner due to static or other transient signals in the system.

Still another object of the present invention is to provide protection for the driving circuit whenever the recording electrode is removed from the recording sheet in an electrolytic or similar facsimile recording device.

A still further object of the present invention to render less critical the setting of the gain controls and other means for determining the recording current in an electrolytic or similar facsimile recording system. An additional object of the present invention is to prevent'excessive current through the recording sheet which might decompose or even set fire to it under conditions which might otherwise yield unduly high recording current.

In the art of facsimile as a present practice a cop sheet is scanned point by point and line by line and density variations are transformed into electrical signals. These electrical signals are amplified and transmitted directly or by means of modulated signals transmitted to remote re- 1 Claim. (Cl. 346-88) recording sheet, there is also a great need for the protection of the output tube or tube's circuit. In practise, one method isto employ a constant current tube as the output control tube for the recording current. A constant current tube is useful to maintain a source of high fidelity recording current on the recording electrodes, reg'ardless of the variations in resistance or conductivity in the recording sheet and the recording circuit. One of the characteristics of a constant I current tube is that variations in the plate load ceiving points. At a receiving point the facsimile signals are utilized to control the recording current passing through or otherwise controlling recording devices which produce density variations in a recording sheet correspondingto the density variations of the original copy. In most facsimile systems the darkest marksproduced in the reviving device correspond to the maximum recording current. In a system employing electrolytic recording sheet the maximum density is secured by a recording current which is close to the maximum amount of current which the sheet w ll pass without disintegration. In the past it has been necessary to closely control the recording current so that maximum density may be secured but at the same time current which may cause disintegration or .burning of the recording sheet may be avoided. The boundary between the current required to produce maximum density of the recorded mark and the current which will produce disintegration of the recording sheet or actually cause it to burn may be very slight.

' In addition to the need for the protection or the circuit over a considerable range have little or no effect on the plate-cathode current flow. This result is usually produced by maintaining a screen grid in the tube at a high positive voltage. When used in recording circuits of the type described herein, it is usual to have the recording electrode, and the recording sheet, in the plate load circuit during a major portion 01' the time. But, because of the need for the phasing and synchronizing of the recorder with the transmitter, the electrodes must be switched out of the plate load circuit during a small portion of a recording cycle. During this period the plate of the constant current tube is swinging free and the screen grid will draw all the current, unless an auxiliary load is used to maintain adequate plate voltage.

It has been found according to the present invention that if a non-linear circuit element is shunted across the recording circuit having the characteristics of a relatively high impedance for voltages up to a certain point and 9. suddenly lower impedance for voltages above this point that the maximum recording current may be passed through the recording sheet but that current beyond this point may he suddenly controlled so that overloading or burning of the sheet may be avoided. Also this non-linear element will act as an automatic volume control rendering gain settings in the system less critical. Normal signals will be substantially unaffected while abnormal signals will becompressed and kept from overloading the recordingsheet. I

It has also been found that the same non-linear circuit element may beused for the'jabove de- I scribed purpose and, in addition, as anauxlliary load to maintain a safe voltage on the plate of the tube during the period when the recording electrodes are not in the plate circuit. A linear type of load may not be used for the same purpose, unless switched into and out of the circuit, but a non-linear resistor, of the proper size, may be permanently installed in the circuit, without affecting the quality of the recording. Thus the same non-linear resistor may be employed to prevent burning of the recording paper during the recording portion 01' the cycle and to prevent the burning out of the constant current tube, during the non-recording portion, of the cycle, both functions being performed without switching or otherwise changing the circuit of the resistor.

Fig. 1 shows a circuit embodying one form of the present invention.

Fig. 2 shows a curve oi the voltage versus current characteristics or a suitable non-linear circuit element for practising the present invention.

Fig. 1 shows a source of facsimile signals 3, having output terminals l and I, feeding output tubes 6 and l. The output circuit may include one or more tubes as for instance the two tubes 8 and 1 connected in parallel as shown. Tubes I and I may be any suitable output tubes but should preferably be constant current tetrodcs, as shown. Tube i has cathode ii heated by conventional means not shown, control grid l0, screen grid I and plate I, while tube 1 has cathode II heated by conventional means not shown, control grid ll, screen grid l3 and plate II. The corresponding elementssuch as cathodes ii and II of the two tubes I and I are connected in par she] to provide more recording current than would be provided by a single tube. The facsimile signals from source I are connected through bias source 2| providing cut-oi! in the absence of signal between cathodes Ii and ii and control grids iii and II, as shown. Screen grids and ii are energized from a source of bias voltage II and plates I and I! are connected to helical electrode ll of the facsimile recorder while the linear electrode II is connected to the positive side of the bias voltage source Ii. Recording is accomplished by passing current between-helical electrode ll as it is rotated and linear electrode ll through the electrolytic recording paper II. Since helical electrode ll does not make acomplete 360 circle, but only a turn of 315, there is a 45" portion oi the recording cycle whenthere is no interaction between linear electrode I. and helical electrode ll. During this period the plate circuit would normally be open except for the presence of the resistor 20. With no plate voltage, excessive current would be-drawn by grids 0 and i3, burning out the tubes. Since the plate current of tubes i and l is under the control of the facsimile signal from source I the current passing through theeiectrolytic recording paper II at any instant is determined by the instantaneous value of the facsimile signal and corresponding density variations will be produced in the recordingsheet. The recording sheet is advanced by conventional means not shown completing the scanning operation. In order to ren- -der the setting ofgain control or other conventional control means in the facsimile receiver less critical, to protect theconstant current tube from being overloaded and to preventoverloading and burning of the electrolytic paper I, a non-linear resistor II is connected between helical electrode I1 and linear electrode "as shown. In order to accomplish the purposes or the. present invention this resistor 20 should have .the characteristic that it has a high resistance for all voltages up to a certain point and a much lower resistance at voltages above this point. The resistor should be so chosen that the point oi transition should approximate the maximum current which it is desired to pass through the recording sheet. An example ofthe type or resistor or non-linear circuit element which is suitable for this purpose is a silicon-carbide combination known by the trade name of Thyrite.

Fig. 2 shows the voltage current characteristics of a suitable non-linear resistor as shown at 20 in Fig. 1. In Fig. 2 it is shown how the resistor characteristic is that of a relatively high resistance for voltages from 0 to E1 at which point the current 11 is passed and that of a. relatively lowresistance from E1 to E: at which point the current oi In is passed. Point E1 should coincide substantially with the maximum voltage which it is desired to place across the recording sheet. Higher voltages will be heavily shunted by the lowering resistance value of the non-linear resistor. As can be seen, during the recording period, except to prevent excessive voltages that might appear on the electrodes, the non-linear resistor does not interfere with the recording process. During the non-recording portion or the cycle, the non-linear resistor becomes the plate load and replaces the electrode load circuit so that the plate always is supplied with adequate voltage. For normal currents, it again appears only as a normal load.

While only one embodiment of the present inventionhas been shown and described many variations'will be apparent to those skilled in the art within the spirit and scope of the invention as set forth in the appended claim.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

In a recorder adapted to record upon a recording medium exhibiting a maximum tolerable recording voltage, a source of signals, at least one constant current vacuum tube for controlling a recording current, input and output circuits to said tube, said input circuit being connected to said signal source, at least one pair of electrodes connected to said output circuit for impressing recording signals upon said recording medium located therebetween, and at least one non-1inear resistor, having a relatively high resistance at voltages below said tolerable voltage and a relatively low resistance at voltages above said tolerable volta e, connected across said electrodes and the output circuit of at least one of said constant current tubes, said non-linear resistance alternately preventing the output voltage of said output circuit from exceeding said maximum tolerable recording voltage and preventing said output voltage from decreasing to a point where low output voltage may injure said constant current tubes.

RICHARD C. CUR'IIS.

REFERENCES CITED UNITED STATES PATENTS Name Date -Tribble July 31, 1945 OTHER REFERENCES Howard, Non-Linear Resistors, Radio, April 1945, pages 29, 30.

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